a trailer with a mass of 150 kg happen to roll away with the speed 20 m/s. 450 kg cement fall staight down into the trailer! which speeds does the trailer get when the cement fall into it?

2. Relevant equations

I know how to solve it but I don't know why it works. Is it because this counts as a collision?

3. The attempt at a solution

150 * 20 = V(150+450)

Momentum is described by a vector, because velocity is a vector. You can bilance the momenta before and after the cement fell on the trailer in two perpendicular directions (x / horizontal & y / vertical) - in both directions momentum has to be conserved.

As the momenta of the two objects can't be the same, a force must have affected them - the earth. Taking into account the momentum of earth (assuming it was standing still, when the cement hit the trailer):